In a synchronous rectification DC-DC converter, a forward voltage drop of a low-side switch is smaller than that in a diode rectification DC-DC converter, and hence the synchronous rectification DC-DC converter is advantageous particularly to enhancement in conversion efficiency of a DC-DC converter with a low output voltage. There is a problem, however, that because an inductor current is wastefully discharged to a ground via a low-side switch at the time of a low load, the efficiency at the time of the low load decreases.
In order to solve this problem, the following technique has been proposed. That is, PWM (Pulse Width Modulation) control is performed in a high load mode, and PFM (Pulse Frequency Modulation) control is performed in a low load mode. Such two control methods are switchingly used, whereby a high efficiency is achieved over a wide load current range.
In the low load mode of this technique, when an output voltage is smaller than 1.017 Vout in terms of an output set voltage Vout, the switching is continued within a range within which the inductor current is not negative. After the output voltage reaches 1.017 Vout, two high-side and low-side switches are both turned off until the output voltage decreases to 1.006 Vout.
In the low load mode, when a load current increases and the output voltage thus decreases to be equal to Vout, the transition is made from the low load mode to the high load mode. In the high load mode, continuous switching is performed according to the PWM control in synchronization with clocks. At this time, a phase compensator is provided in a control loop in order to secure the stability.
Unfortunately, according to the above-mentioned method, the output voltage needs to be compared with three reference voltages of Vout, 1.006 Vout, and 1.017 Vout in order to perform the PFM operation, and this requires three high-precision comparators.
Further, the switching in the low load mode is controlled such that the inductor current is equal to or more than zero and equal to or less than a predetermined value, and this requires real-time observation of the inductor current value.
Still further, a response when the load current increases in the low load mode and the transition is thus made to the high load mode is determined according to the PWM control, and because the response speed is limited by the phase compensator provided in the PWM control loop, the output voltage temporarily decreases to a large extent with respect to a desired value.